This invention relates to adhesive compositions, especially aqueous-based contact adhesive compositions. The adhesive finds particularly preferred use for bonding substrates such as high-pressure laminate (HPL), to particleboard for kitchen counter tops in the decorative laminate industry.
In the past decade, environmental considerations have forced reevaluation of many established products. Contact adhesives based on elastomeric polymers, which have been available commercially for several decades, fall into this category.
The term contact adhesive refers to an adhesive which is typically applied to two surfaces to be adhered together, dried to form film adhesive layers, and brought into contact with pressure so as to form an immediate, durable bond. Once the contact adhesive is pressed together, the adhered pieces are ready for further processing such as cutting.
Many contact adhesive applications require that soon after applying the adhesive to the substrates to be contacted that the adhesive film dries and forms a film within five minutes and thereafter pressing the coated substrates together, the adhesive rapidly bonds to itself. That is, the adhesive undergoes auto adhesion and forms a semi-solid film with sufficient strength to hold the substrates together and resist subsequent forces on the fresh bond line that might cause failure. Adhesives that provide a dried adhesive film within five minutes and develops handling strength properties immediately after bonding without adhesive squeeze out can be referred to as xe2x80x9cfast setxe2x80x9d contact adhesives. Quick-drying flammable and chlorinated organic solvents have been conventionally utilized as carriers for fast-set contact adhesives since their quick drying film forming properties facilitate quick holding together of the bonded parts after their coated surfaces are mated and pressed together within five minutes after applying to the substrate.
Flammable and chlorinated solvent-based contact adhesives have traditionally dominated the high-pressure laminate industry. The low boiling point organic solvents used as carriers for contact adhesives provide the advantage of rapid dry times, whereupon the workpieces may be contact adhered within five minutes after application of the adhesive compositions. While such products have been used both in consumer and industrial applications, generally the use of solvent-based systems has declined as a result of concerns related to the presence of volatile and organic solvents. It is, therefore, a goal of the adhesive industry to develop alternative contact adhesives, which exhibit fast set and physical properties equivalent to those of solvent-based adhesives, but wherein the amount of organic solvent is substantially reduced or completely eliminated.
Most of the commercially available water-based contact adhesives are based on polychloroprene and/or acrylic/vinyl acrylic type latices, typically in combination with materials such as resins, plasticizers, antioxidants, and other ingredients commonly used in adhesive compositions (see U.S. Pat. No. 5,543,455). The current one-part water-based adhesives have experienced only gradual industry acceptance due to longer dry times and relatively slow rate of strength build. (See U.S. Pat. No. 5,264,467, showing a one hour dry time). To overcome such limitations, two-part, co-sprayed, water-dispersed, high solid content adhesive systems have been developed that demonstrate similar dry times to solvent-based adhesives and provide strength build within seconds. (See DuPont Dow Elastomer, Technical Information, AquaStik(trademark), xe2x80x9cWater Based Adhesive for Furniture Foam: Quick Break Adhesives.xe2x80x9d) Current two-part adhesives typically consist of a bulk adhesive and an external coagulant. Typically an external coagulant based on an acid such as citric, lactic or acetic acid, or salts such as zinc sulfate are used as a co-spray in predetermined ratio. Such two-part adhesive systems are not entirely satisfactory because the equipment is expensive and requires regular maintenance as well as the need to monitor the ratio of coagulant to base adhesive.
Fast-set adhesives have not been obtained without decreasing storage stability at room and elevated temperatures. To date, fast set times and storage stability has not been satisfactorily obtained within the same adhesive composition in a one-part system. The problem has been that in order to achieve a fast-setting adhesive composition, it is necessary to find some additive or additives to make the resulting adhesive composition sensitized and unstable enough that the colloid polymer system immediately xe2x80x9cbreaksxe2x80x9d on application and forms a high strength film, but yet does not make the composition unstable to the point that it will coagulate during storage and before application. Such additives are sometimes referred to as internal coagulants or gelling agents.
Additives used to sensitize anionic-stabilized latex compounds and improve their ability to break upon application are often acidic. The reaction of the acid is immediate depending upon the type of acid. Therefore, the acid must be added at the point of application. For example, Carl, J. C., xe2x80x9cFluid Properties,xe2x80x9d Neoprene Latex: Principles of Compounding and Processing, E.I. DuPont de Nemours and Co., p. 18-19 (1962) discusses the use of sodium and potassium silicofluorides.
Amino acids, such as glycine, do not coagulate neoprene latex but do destabilize by reducing pH. For example, in the production of neoprene foam and dipped goods, pH reduction with glycine is advantageous. Glycine shortens the interval between the addition of gelling agent and the start of coagulation. Anionic neoprene latex compounds are unstable at a pH of less than 10.5.
To comply with consumer demand, it would be desirable to find a storage-stable one-part aqueous-based adhesive that facilitates fast set similar to solvent-based adhesives. Initially it was found that by lowering the pH of anionically stabilized, high crystallinity, low gel content polychloroprene latices, faster set would result. Adjusting the latex pH to 9.5-10.5 maintains an adequate shelf and pumping stability but sufficiently destabilizes the latex to create fast breaking properties on spraying. However, the resulting fast breaking adhesive demonstrates an inadequate immediate bonding capability with fairly low strength characteristics. Further, lowering the pH results in poor colloidal stability at room and elevated temperature and thus loss of formulating latitude with typical materials used in polychloroprene adhesive compositions. (See PCT WO 98/53019).
Another approach to fast break technology is to formulate a polymeric latex blend with relatively low shear stability, which can be spray-applied but coagulates very quickly through moisture loss. In this particular approach, fast crystallizing and/or low molecular weight polychloroprene lattices whose pH ranges are between 10 and 13, are blended with an acrylate dispersion whose pH range is between 3 and 5. The acrylate dispersion provides xe2x80x9csome initial tack and also has a destabilizing effect, which accelerates coagulationxe2x80x9d. (See Gerlach, Dieter, xe2x80x9cPolychloroprene-An Evergreen Product for the Formulation of Water Based Contact Adhesives,xe2x80x9d Advances in Adhesives and Sealants Technology, paper 14). Adhesives formulated with this technology typically show very poor storage stability and mechanical instability.
Thus, there is a need for an aqueous-based polychloroprene contact adhesive that is available in one part. The contact adhesive must be storage stable, yet capable of fast setting. It is also desirable that the contact adhesive be capable of developing enough strength for demanding applications within a short period of time.
In one aspect, the invention provides a stabilized, one-part, water-based contact adhesive composition that comprises an adhesive component comprising polychloroprene rubber, a resinous component, and an internal coagulant. The resinous component is selected from ethylene-vinyl acetate copolymer, polyvinyl acetate homopolymer, polyvinyl acetate copolymer, acrylic copolymer, polyvinyl acetate acrylic copolymer, and combinations thereof. The polychloroprene rubber component may comprise a mixture of polychloroprene rubbers, a mixture of polychloroprene rubber and natural rubber, synthetic rubber, or combinations thereof, and make up the adhesive component of the composition. The internal coagulant is a hygroscopic salt, preferably in an amount of about 0.06 percent to about 6 percent, more preferably, about 0.3 percent to about 1.2 percent by weight based on dry weight of the total adhesive component.
The composition can also contain an amino acid as an internal coagulant, such as glycine, preferably in an amount of about 0.1 percent to about 12 percent by weight based on dry weight of the total adhesive component, and even more preferably in an amount of about 1 percent to about 6.5 percent by weight based on the dry weight of the total adhesive component.
In another aspect, the invention provides a method of bonding two substrates together comprising the steps of applying an adhesive composition comprising polychloroprene rubber, a resinous component selected from the group consisting of ethylene-vinyl acetate copolymer, polyvinyl acetate homopolymer, polyvinyl acetate copolymer, acrylic copolymer, polyvinyl acetate acrylic copolymer, and combinations thereof, and an internal coagulant, to a surface of both substrates, allowing the adhesive composition to dry and contacting the adhesive applied surfaces together. By xe2x80x9cdryxe2x80x9d, it is meant that there is no adhesive transfer when light finger pressure is applied to the adhesive composition in film form.
In another aspect, the invention provides a stabilized, one-part, water-based contact adhesive composition that comprises an adhesive component comprising polychloroprene rubber and an internal coagulant wherein the internal coagulant is a mixture of a hygroscopic salt and an amino acid. The amount of hygroscopic salt and amino acid used in combination in the composition is additive. Thus, the composition can contain both hygroscopic salt and amino acid preferably in an amount of about 0.16 percent to about 18 percent, more preferably about 1.3 percent to about 7.7 percent based on the dry weight of the total adhesive component.
xe2x80x9cShelf lifexe2x80x9d refers to the time period after which the aqueous composition has substantially coagulated, congealed, curdled, separated, settled, or formed non-easily mixable or non-easily dispersible layers so that it may not be usefully or readily applied as a homogeneous, uniform liquid blend by spray-coating methods. That is, compositions that have good shelf life are storage stable. By xe2x80x9cstorage stable,xe2x80x9d it is meant that the aqueous compositions have a shelf life of greater than about four months when stored at room temperature (25xc2x0 C. S.T.P.). Preferably, the aqueous compositions have a shelf life of greater than about 6 months, and more preferably greater than about 8 months when stored at room temperature.
xe2x80x9cFast-settingxe2x80x9d means that the adhesive dries (as defined above) and forms a film within 5 minutes after application. Preferably, fast-setting contact adhesive compositions of the invention form a dried film adhesive in less than 5 minutes after application. For some utilities, particularly when bonding high pressure laminate to counter tops in the laminate industry, fast-setting contact adhesive compositions of the invention preferably dries within 3 minutes or less after application. After pressing the coated substrates together, the adhesive provides sufficient overlap shear strength to hold the substrates together and generally have a minimum strength of about 10 to about 20 psi (about 68.9 to about 138 Pa).
The adhesive compositions are fast setting due to the low pH of the compositions. It is preferred that the compositions have a pH in the range of about 8 to about 12. More preferably, the pH of the compositions is about 8.0 to about 10.5, even more preferably from about 9.0 to about 10.0, and most preferably from about 9.2 to 9.6.
Aqueous-based adhesive compositions according to the present invention comprise polychloroprene in the form of an aqueous dispersion. By xe2x80x9caqueousxe2x80x9d it is meant that the carrier is primarily water. However, organic solvents may be present, so long as they do not substantially compromise storage stability of the composition. Incidental solvents, such as those present in additives and commercially available components may also be present. Preferably, however, xe2x80x9caqueousxe2x80x9d refers to a 100 percent water carrier.
Polychloroprene, as used herein, refers to a homopolymer or copolymer of chloroprene (2-chloro-1,3-butadiene). Co-monomers for use in copolymers include a wide variety of compounds such as styrene, vinyl toluene, 2,3-dichlorobutadiene, acrylic acid, methacrylic acid, and derivatives such as methacrylates, acrylates, and acrylonitriles.
Polychloroprene is readily available as an aqueous dispersion, such as a latex or emulsion. While a broad range of dispersions are suitable, useful commercially available dispersions generally have from about 30 percent to about 65 percent, more usually from about 45 percent to about 60 percent, preferably from 40 percent to 50 percent or 60 percent by weight solids content. The dispersions generally have a particle size in the range of from about 0.1 microns in diameter to about 0.4 microns in diameter, depending on the specific grade.
Polychloroprene dispersions are typically stabilized to prevent the dispersion from coagulating during storage. By stabilization, it is meant that the pH is raised to prevent coagulation due to a decrease in the pH. As the pH of the dispersion drops to 10 or below, there is a tendency to coagulate. The pH tends to drop gradually on storage. Therefore, as mentioned above, commercially available polychloroprene dispersions often include a strong alkali, such as potassium hydroxide, and may have a pH any where from about 10 to about 13, but commonly from about 12 to about 13.
Any suitable stabilizer may be utilized. For example, surfactants may be added to stabilize the dispersion, usually anionic or nonionic surfactants. Thus, a dispersion referred to as an anionic surfactant-stabilized polychloroprene is a polychloroprene dispersion stabilized by an anionic surfactant. Anionic surfactant-stabilized polychloroprene dispersions or a blend of anionic/nonionic surfactant-stabilized polychloroprene dispersions are preferred, however, for the present compositions.
Suitable anionic surfactant-stabilized polychloroprene dispersions include those available from DuPont Dow Elastomers Llc, Wilmington, Del., under the tradenames: LATEX 750 (xe2x80x9c750xe2x80x9d); and AQUASTIK GRADES AQS2920, AQS2540, AQS2126, and AQS9426 (xe2x80x9c2920xe2x80x9d, xe2x80x9c2540xe2x80x9d, xe2x80x9c2126xe2x80x9d, and xe2x80x9c9426xe2x80x9d). Another suitable anionic surfactant-stabilized polychloroprene dispersion is DISPERCOLL C74, available from Bayer Corporation, Pittsburgh, Pa. (xe2x80x9cC74xe2x80x9d). A suitable anionic/nonionic surfactant-stabilized polychloroprene blend dispersion is available from Bayer Corporation under the tradename DISPERCOLL C84 (xe2x80x9cC84xe2x80x9d). A suitable sol polymer polychloroprene dispersion is available from Dupont Dow Elastomers Llc under the tradename LATEX 735 (xe2x80x9c735xe2x80x9d).
Polychloroprene is available in a wide variety of types, from low to high crystallinity, and from low to medium or high gel. The type of polychloroprene used can influence the ability of the composition to achieve a fast set time and other properties of the resulting bond. The effects from the choice of polychloroprene can be seen in the examples following.
Besides polychloroprene, the adhesive composition may also contain a resinous component selected from vinyl acetate ethylene emulsion (VAE), polyvinyl acetate homopolymer emulsion, polyvinyl acetate copolymer emulsion, acrylic copolymer emulsion, polyvinyl acetate acrylic emulsion, or combinations thereof. Preferably, the commercially available emulsions are 40 to 65 percent by weight solid content. Suitable polyvinyl acetate ethylene emulsion and polyvinyl acetate acrylic emulsion include those available from Air Products and Chemicals, Inc, Allentown, Pa., under the tradenames: AIRFLEX 405, AIRFLEX 410, AIRFLEX 420, and AIRFLEX 465; and FLEXBOND 150, FLEXBOND 153, FLEXBOND 165 and FLEXBOND 825, respectively. Other suitable polyvinyl acetate acrylic emulsions are those having the tradenames ROVACE 6930 and ROVACE 3270, available from Rohm and Haas Company, Philadelphia, Pa. A suitable polyvinyl acetate homopolymer emulsion is available from Air Products and Chemicals, Inc under the tradename VINAC 285. The resinous component may be present in the adhesive compositions of the invention at a level of from about 3.5 to about 30 parts per hundred (phr), preferably, about 5 to about 20 phr, more preferably, about 7.5 to about 15 phr based on the dry weight of the adhesive component.
The adhesive compositions of the invention also contain an internal coagulant. The function or purpose of an internal coagulant is to enhance dry film formation. A useful internal coagulant is a fully neutralized hygroscopic salt; that is, a salt in which all exchangeable protons has been replaced by cations. Examples of useful hygroscopic salts include hygroscopic phosphates and sulfates. Preferred hygroscopic salts include Li, Na, and K sulfates and phosphates. Preferred salts include tri-sodium phosphate and di-sodium sulfate. The hygroscopic salts may be anhydrous or partially hydrated.
Another useful internal coagulant is an amino acid. Useful amino acids include water soluble, lower molecular weight amino acids such as glycine, alanine, valine, lysine, iso-leucine, leucine, and combinations thereof, with glycine being preferred. Preferably, the amino acid is used in the form of a solution containing from about 5 percent to about 25 percent by weight, more preferably about 10 percent to about 18 percent by weight of amino acid.
Preferably, the adhesive compositions of the invention contain a mixture of a hygroscopic salt and an amino acid. A preferred combination is a hygroscopic phosphate and glycine. Generally the ratio of hygroscopic salt to amino acid ranges from about 5:1 to about 1:5 by weight and preferably from about 2:1 to about 1:2 by weight.
The adhesive component of the compositions of the invention may also contain natural rubber, another synthetic rubber, or combinations thereof. Preferably, such natural and synthetic rubbers have unsaturated chains derived from units such as butadiene or have units derived from C4 to C10 conjugated dienes, such as styrene or methyl methacrylate. Natural rubber is commercially available as modified general-purpose latex. For example, modified latex is available under the tradenames MEGAPOLY latex, as MG 49, and as MG 30 (rubber latices grafted with methyl methacrylate and available from H. A. Astlett and Co., Inc., Toronto, Ontario). Other synthetic rubbers include homopolymers of butadiene, isoprene, or dimethyl butadiene. Other useful synthetic rubbers include copolymers of butadiene and styrene, isoprene and styrene, butadiene and dimethyl butadiene, butadiene and acrylonitrile, isoprene and acrylonitrile, dimethyl butadiene and styrene, butadiene and vinyl toluene, or isoprene and vinyl toluene. A suitable commercially available synthetic rubber is ROVENE 8329 (modified styrene butadiene rubber latex emulsion, available from Mallard Creek Polymers, Inc, Charlotte, N.C.).
More than one type of polychloroprene can be blended together to form the polychloroprene rubber component. Blending is permitted so long as the aggressiveness or initial bond strength of the compositions is not significantly compromised. If polychloroprene and a natural rubber are blended, it is preferred that the natural rubber ranges from about 10 percent to about 90 percent by weight, more preferably from about 30 percent to about 60 percent by weight based on the dry weight of the adhesive component.
One or more tackifiers may be used in the contact adhesives of the invention. Tackifiers can be used at levels of up to about 50 percent by weight, preferably about 30 percent by weight, and more preferably about 5 percent by weight to about 20 percent by weight based on the dry weight of the total adhesive component. About 25 to about 60 phr based on the dry weight of the total adhesive component would also be suitable. Suitable tackifiers for use in the contact adhesives of the invention include rosin acids, rosin esters, terpene phenolic resins, hydrocarbon resins, and cumarone indene resins. The tackifier will generally be used in the form of an aqueous dispersion.
Typically, about 1 to about 6 phr of acid acceptor may be added based on dry weight of the total adhesive component to minimize the degradation of the adhesive composition due to release of hydrochloric acid from the polychloroprene rubber. Zinc oxide dispersions are available from Tiarco Chemical Division, Dalton, Ga., under the tradename OCTOCURE 462 and 803. Aqueous epoxy resins, such as those having the tradenames EPI-REZ 3519, EPI-REZ 3515 and EPI-REZ 5003 (available from Shell Chemical Company, Houston Tex.) may also be used.
Viscosity modifiers may be added to thicken the adhesive compositions of the invention. Suitable thickeners include hydrophobically modified, alkali-soluble, acrylic emulsion copolymers, such as those having the tradenames ACRYSOL RM5 and ACRYSOL ASE 95 (available from Rohm and Haas Company), hydrophobically modified urethanes, such as those having the tradenames NOPCO DSX 1550 (available from Henkel Canada Ltd., Mississauga, Ontario), and nonionic, water soluble polymers derived from cellulose, cellulose ethers and carboxymethyl celluloses, such as those having the tradename NATROSOL 250 (a hydroxyethylcellulose, available from Hercules Incorporated, Wilmington, Del.), or inorganic thickeners, such as fumed silica. Thickeners can normally be used at up to about 1 percent by weight of the dry weight of the total adhesive component.
Uncompounded polychloroprene latex generally has good mechanical and storage stability, but the composition may require the incorporation of additional surfactants, wetting agents, or stabilizers. Surfactants, such as MODICAL S (a sulfonated fatty product, available from Henkel Adhesives Corporation, Elgin, Ill.), EMULVIN W (aromatic polyglycol ether, available from Bayer Corporation), and DARVAN WAQ (sodium lauryl sulfate, available from R. T. Vanderbilt Company, Inc., Norwalk, Conn.) are suitable. Typically they would be added at up to about 2 phr based on dry weight of the total adhesive component.
Pigments may be added to color the adhesive compositions of the invention. Suitable pigments are available as powders, which are water dispersible, or as aqueous dispersions. Some suitable pigments include Phthalocyanine Green and Blue Pigment Dispersion (both available from Hilton Davis Inc, Cincinnati, Ohio.), Orange Pigment Aqueous Dispersion (available from Engelhard Corporation, Iselin, N.J.), and Carbon Black Aqueous Dispersion (available from Technical Industries Inc., Peace Dale, R.I.). Typically, pigments may be added at up to about 0.2 phr based on dry weight of the total adhesive component.
Preservatives, such as biocides, may be added to prevent such storage problems as bacterial or fungal attack. Suitable preservatives include having the tradenames TROYSAN 586 (available from Troy Corp., Florham Park, N.J.) and VANCIDE 51 (available from R. T. Vanderbilt Company, Inc.). Typically, such compounds may be added at about 500 parts per million (ppm) to about 1500 ppm based on the wet weight of the total adhesive component.
The aqueous contact adhesive compositions can be prepared by mixing the ingredients together at room temperature. Normal, low shear mixing equipment can be used. Hygroscopic salt and amino acid (if any), and other additives (if any), are preferably added as aqueous solutions, if available.
The adhesive compositions can be prepared without any prior pH adjustment of the polychloroprene used or of any other resinous component that may be present. The pH of the finished composition is generally in the range of from about 8 to about 12. Suitable natural and synthetic rubbers have high pH values and are, therefore, compatible with polychloroprene. Adhesive components that have a pH in the range of from about 9.5 to about 13 are generally compatible with the polychloroprene latex and, thus, facilitate formulation.
When it is desired to use the composition, both surfaces to be adhered are coated with the adhesive composition and brought into contact with at least light hand pressure. The composition can be applied to the surfaces by any suitable method, such as brushing, spraying, or rolling. Preferably and conveniently, the composition is applied to the surfaces by spray coating. Unlike conventional two-part systems, the one-part contact adhesive of the present invention can be sprayed from one container, without the need for expensive equipment and monitoring of the ratio of components. Usually coverage is about 1 gram to about 10 grams (dry weight) per square foot (11.1 grams to 111 g/m2) of surface to be adhered, preferably about 4 grams to about 6 grams per square foot (44.4 grams to 66.7 g/m2). Suitable spray-coating equipment includes manual spray operators and automated spray operators. Suitable manual spray operators include the BINKS 2001 SS (available from Binks Manufacturing Co, Franklin Park, Ill.), BINKS MACH 1, DEVILBISS MSA-503 (available from DeVilbiss Industrial Spray Equipment, Maumee, Ohio.), GRACO 800N and HVLP GRACO OPTIMIZER (available from Graco Inc., Minneapolis, Minn.). Suitable automated spray operators include the BINKS 61, DEVILBISS AGX-4303, GRACO A800N, and BINKS MACH 1 HVLPP.
The adhesive compositions of the present invention are used to provide a contact adhesive for a variety of substrates. The materials bonded together can be the same or of different compositions. Suitable substrates include wood; metal, such as cold-rolled steel, galvanized steel, and aluminum; fabric; paper; foam; plastic, such as polyvinylchloride, polystyrene, polyethylene, polypropylene, and acrylonitrile butadiene styrene (ABS); fiberglass; and materials used to construct high pressure laminates, for example, for counter tops.